Transforming the Thermal Grill Effect by Crossing the Fingers

The relation between pain perception and spatial representation of the body is poorly understood. In the thermal grill illusion (TGI), alternating non-noxious warm and cold temperatures cause a paradoxical, sometimes painful, sensation of burning heat. We combined thermal grill stimulation with crossing the fingers to investigate whether nociceptively mediated sensation depends on the somatotopic or spatiotopic configuration of thermal inputs. We stimulated the index, middle, and ring fingers when the middle finger either was or was not crossed over the index to generate "warm-cold-warm" patterns in either somatotopic or spatiotopic coordinates. Participants adjusted a temperature delivered to the other hand until it matched their perception of the cold target finger (index or middle). We found significant temperature overestimation when the target was central within the spatial configuration (warm-cold-warm) compared to when it was peripheral (cold-warm-warm). Crucially, this effect depended on the spatiotopic configuration of thermal inputs, but it was independent of the finger posture and present for both index and middle target fingers—the thermal grill effect for the middle finger was abolished when it was crossed over the index to adopt a spatiotopically peripheral position, while the same effect was newly generated for the index finger by the same postural change. Our results suggest that the locations of multiple stimuli are remapped into external space as a group; nociceptively mediated sensations depended not on the body posture, but rather on the external spatial configuration formed by the pattern of thermal stimuli in each posture.